Tuning the Magnetic Anisotropy of Lanthanides on a Metal Substrate by Metal-Organic Coordination.

Taming the magnetic anisotropy of lanthanides through coordination environments is crucial to take advantage of the lanthanides properties in thermally robust nanomaterials. In this work, the electronic and magnetic properties of Dy-carboxylate metal-organic networks on Cu(111) based on an eightfold coordination between Dy and ditopic linkers are inspected. This surface science study based on scanning probe microscopy and X-ray magnetic circular dichroism, complemented with density functional theory and multiplet calculations, reveals that the magnetic anisotropy landscape of the system is complex. Surface-supported metal-organic coordination is able to induce a change in the orientation of the easy magnetization axis of the Dy coordinative centers as compared to isolated Dy atoms and Dy clusters, and significantly increases the magnetic anisotropy. Surprisingly, Dy atoms coordinated in the metallosupramolecular networks display a nearly in-plane easy magnetization axis despite the out-of-plane symmetry axis of the coordinative molecular lattice. Multiplet calculations highlight the decisive role of the metal-organic coordination, revealing that the tilted orientation is the result of a very delicate balance between the interaction of Dy with O atoms and the precise geometry of the crystal field. This study opens new avenues to tailor the magnetic anisotropy and magnetic moments of lanthanide elements on surfaces.

Electron Accumulative Molecules.

With the goal to produce molecules with high electron accepting capacity and low reorganization energy upon gaining one or more electrons, a synthesis procedure leading to the formation of a B-N(aromatic) bond in a cluster has been developed. The research was focused on the development of a molecular structure able to accept and release a specific number of electrons without decomposing or change in its structural arrangement. The synthetic procedure consists of a parallel decomposition reaction to generate a reactive electrophile and a synthesis reaction to generate the B-N(aromatic) bond. This procedure has paved the way to produce the metallacarboranylviologen [M(C2B9H11)(C2B9H10)-NC5H4-C5H4N-M'(C2B9H11)(C2B9H10)] (M = M' = Co, Fe and M = Co and M' = Fe) and semi(metallacarboranyl)viologen [3,3'-M(8-(NC5H4-C5H4N-1,2-C2B9H10)(1',2'-C2B9H11)] (M = Co, Fe) electron cumulative molecules. These molecules are able to accept up to five electrons and to donate one in single electron steps at accessible potentials and in a reversible way. By targeted synthesis and corresponding electrochemical tests each electron transfer (ET) step has been assigned to specific fragments of the molecules. The molecules have been carefully characterized, and the electronic communication between both metal centers (when this situation applies) has been definitely observed through the coplanarity of both pyridine fragments. The structural characteristics of these molecules imply a low reorganization energy that is a necessary requirement for low energy ET processes. This makes them electronically comparable to fullerenes, but on their side, they have a wide range of possible solvents. The ET from one molecule to another has been clearly demonstrated as well as their self-organizing capacity. We consider that these molecules, thanks to their easy synthesis, ET, self-organizing capacity, wide range of solubility, and easy processability, can find important application in any area where ET is paramount.

Heterogeneous nanotribological response of polymorphic self-assembled monolayers arising from domain and phase dependent friction.

Micro-/nanoelectromechanical systems demand robust ultrathin films for lubrication. As they can drastically modify the frictional properties of surfaces, few nanometers thick self-assembled monolayers (SAMs) constitute accepted candidates as boundary lubricants. Their high stability and easy preparation make them attractive also for low cost applications. Given their high order, organosulfur SAMs have been archetypal systems for structural investigations, but few efforts have been devoted to analyze the influence of lateral inhomogeneities on their surface properties. The impact on the frictional response of the surface due to the existence of crystalline domains with lateral dimension in the sub-micrometer range is considered here. To this end, two polymorphic structures of self-assembled monolayers of ω-(4'-methylbiphenyl-4-yl) butane-1-thiol coexisting on Au(111) are investigated by scanning tunneling and force microscopy. Described by rectangular 5√5 × 3 (α-phase) and oblique 6√3 × 2√3 (ß-phase) unit cells, they exhibit pronouncedly different frictional responses. The lateral nano-tribological heterogeneity of the surface is further influenced by the azimuthal orientation dependence of friction for each phase. In particular, this phenomenon is exploited in the less densely packed ß-phase for which the separate analysis of forward and backward lateral force scans is used to differentiate domains rotated 180°. The results demonstrate the level of structural control required in the design of SAMs for nano-tribology applications.

Tuning the supramolecular chirality of one- and two-dimensional aggregates with the number of stereogenic centers in the component porphyrins.

A synthetic strategy was developed for the preparation of porphyrins containing between one and four stereogenic centers, such that their molecular weights vary only as a result of methyl groups which give the chiral forms. The low-dimensional nanoscale aggregates of these compounds reveal the profound effects of this varying molecular chirality on their supramolecular structure and optical activity. The number of stereogenic centers influences significantly the self-assembly and chiral structure of the aggregates of porphyrin molecules described here. A scanning tunneling microscopy study of monolayers on graphite shows that the degree of structural chirality with respect to the surface increases almost linearly with the number of stereogenic centers, and only one handedness is formed in the monolayers, whereas the achiral compound forms a mixture of mirror-image domains at the surface. In solution, four hydrogen bonds induce the formation of an H-aggregate, and circular dichroism measurements and theoretical studies indicate that the compounds self-assemble into helical structures. Both the chirality and stability of the aggregates depend critically on the number of stereocenters. The chiral porphyrin derivatives gelate methylcyclohexane at concentrations dependent on the number and position of chiral groups at the periphery of the aromatic core, reflecting the different aggregation forces of the molecules in solution. Increasing the number of stereogenic centers requires more material to immobilize the solvent, in all likelihood because of the greater solubility of the porphyrins. The vibrational circular dichroism spectra of the gels show that all compounds have a chiral environment around the amide bonds, confirming the helical model proposed by calculations. The morphologies of the xerogels (studied by scanning electron microscopy and scanning force microscopy) are similar, although more fibrous features are present in the molecules with fewer stereogenic centers. Importantly, the presence of only one stereogenic center, bearing a methyl group as the desymmetrizing ligand, in a molecule of considerable molecular weight is enough to induce single-handed chirality in both the one- and two-dimensional supramolecular self-assembled structures.

Controlling interpenetration in metal-organic frameworks by liquid-phase epitaxy.

Metal-organic frameworks (MOFs) are highly porous materials generally consisting of two building elements: inorganic coupling units and organic linkers. These frameworks offer an enormous porosity, which can be used to store large amounts of gases and, as demonstrated in more recent applications, makes these compounds suitable for drug release. The huge sizes of the pores inside MOFs, however, also give rise to a fundamental complication, namely the formation of sublattices occupying the same space. This interpenetration greatly reduces the pore size and thus the available space within the MOF structure. We demonstrate here that the formation of the second, interpenetrated framework can be suppressed by using liquid-phase epitaxy on an organic template. This success demonstrates the potential of the step-by-step method to synthesize new classes of MOFs not accessible by conventional solvothermal methods.

Tuning the local frictional and electrostatic responses of nanostructured SrTiO(3)-surfaces by self-assembled molecular monolayers.

Exploiting the capability of preparing nanostructured bifunctional terminated SrTiO(3) substrates (SrO and TiO(2)), the surface properties have been locally tuned by employing a double bottom-up strategy which combines the use of chemically nanopatterned substrates with molecular self-assembly. The dynamics of surface diffusion that allows SrO and TiO(2) chemical-termination nano-patterning of the SrTiO(3) is first addressed. Second, termination-dependent heterogeneous nucleation is used to demonstrate that stearic acid selectively grows on the TiO(2) terminated terraces. This adsorption improves the frictional properties and modifies the surface contact potential. The possibility of simultaneously tailoring at the nanoscale different surface properties of widely employed oxide substrates is promising for building up new devices useful for emerging applications.

Layer-by-layer electropeeling of organic conducting material imaged in real time.

Soft materials comprising low-molecular-weight organic molecules are attracting increasing interest because of their importance in the development of a number of emerging areas in nanoscience and technology, including molecular electronics, nanosystems for energy conversion, and devices in the widest sense. Their interaction with electrodes and their behavior under electric fields is a topic of vital significance for these areas, and about which very little is known. Here unprecedented evidence is presented for the controlled peeling of organic molecular material when a voltage is applied between the conducting system and the conducting probe of a scanning force microscope. The rate of removal of the material from the surface of the bulk conducting supramolecular material can be tuned. It depends on the potential applied and is initiated only above a threshold value of 200 mV. The results indicate the importance of electric fields on the stability and performance of conducting organic systems at the nanoscale.

In-Situ Scrutiny of the Relationship between Polymorphic Phases and Properties of Self-Assembled Monolayers of a Biphenyl Based Thiol.

Two polymorphic phases of ω-(4'-methylbiphenyl-4-yl) butane-1-thiol (BP4) molecules formed on Au(111) were investigated by multidimensional atomic force microscopy, combining conductivity measurements, electrostatic characterization, friction force mapping, and normal force spectroscopy. Based on the same molecular structure but differing in molecular order, packing density, and molecular tilt, the two phases serve as a test bench to establish the structure-property relationships in self-assembled monolayers (SAMs). From a detailed analysis of the charge transport and electrostatics, the contributions of geometrical and electronic effects to the tunneling are discussed.

Real Space Demonstration of Induced Crystalline 3D Nanostructuration of Organic Layers.

The controlled 3D nanostructuration of molecular layers of the semiconducting molecules C22H14 (pentacene) and N,N'-dioctyl-3,4,9,10-perylene tetracarboxylic diimide (PTCDI-C8) is addressed. A tip-assisted method using atomic force microscopy (AFM) is developed for removing part of the organic material and relocating it in up to six layer thick nanostructures. Moreover, unconventional molecular scale imaging combining diverse friction force microscopy modes reveals the stacking sequence of the piled layers. In particular, we unambiguously achieve epitaxial growth, an issue of fundamental importance in thin film strategies for the nanostructuration of more efficient organic nanodevices.

On-surface synthesis of superlattice arrays of ultra-long graphene nanoribbons.

We report the on-surface synthesis of graphene nanoribbon superlattice arrays directed by the herringbone reconstruction of the Au(111) surface. The uniaxial anisotropy of the zigzag pattern of the reconstruction defines a one dimensional grid for directing the Ullmann polymerization and inducing periodic arrays of parallel ultra-long nanoribbons (>100 nm), where the periodicity is varied with coverage at discrete values following a hierarchical templating behavior.

Bottom-up synthesis of multifunctional nanoporous graphene.

Nanosize pores can turn semimetallic graphene into a semiconductor and, from being impermeable, into the most efficient molecular-sieve membrane. However, scaling the pores down to the nanometer, while fulfilling the tight structural constraints imposed by applications, represents an enormous challenge for present top-down strategies. Here we report a bottom-up method to synthesize nanoporous graphene comprising an ordered array of pores separated by ribbons, which can be tuned down to the 1-nanometer range. The size, density, morphology, and chemical composition of the pores are defined with atomic precision by the design of the molecular precursors. Our electronic characterization further reveals a highly anisotropic electronic structure, where orthogonal one-dimensional electronic bands with an energy gap of ∼1 electron volt coexist with confined pore states, making the nanoporous graphene a highly versatile semiconductor for simultaneous sieving and electrical sensing of molecular species.

Microfluidic Pneumatic Cages: A Novel Approach for In-chip Crystal Trapping, Manipulation and Controlled Chemical Treatment.

The precise localization and controlled chemical treatment of structures on a surface are significant challenges for common laboratory technologies. Herein, we introduce a microfluidic-based technology, employing a double-layer microfluidic device, which can trap and localize in situ and ex situ synthesized structures on microfluidic channel surfaces. Crucially, we show how such a device can be used to conduct controlled chemical reactions onto on-chip trapped structures and we demonstrate how the synthetic pathway of a crystalline molecular material and its positioning inside a microfluidic channel can be precisely modified with this technology. This approach provides new opportunities for the controlled assembly of structures on surface and for their subsequent treatment.

Bottom-up on-crystal in-chip formation of a conducting salt and a view of its restructuring: from organic insulator to conducting "switch" through microfluidic manipulation.

The chemical modification of an immobilized single crystal in a fluid cell is reported, whereby a material with switching functions is generated in situ by generating a chemical reagent in the flow. Crystals of the insulating organic crystal of TCNQ (tetracyanoquinodimethane) were grown in a microfluidic channel and were trapped using a pneumatic valve, a nascent technique for materials manipulation. They were subsequently reduced using solution-deposited silver to provide a conducting material in situ by a heterogeneous reaction. Removal of the new material from the chip proved it to be the silver salt of reduced TCNQ. Uniquely, conducting atomic force microscope (CAFM) studies show three regions in the solid. The localized original neutral organic material crystal is shown to be an insulator but to produce areas with Ohmic conducting characteristics after reduction. This inhomogeneous doping provides an opportunity for probing electrical materials properties side by side. Measurements with the CAFM witness this conducting material where the TCNQ is fully transformed to the silver salt. Additionally, an intermediate phase is observed that exhibits bipolar resistive switching typical of programmable resistive memories. Raman microscopy proves the conversion of the material in specific regions and clearly defines the intermediate phase region that could be responsible for the switching effect in related materials. This kind of "on crystal chemistry" exploiting immobilization and masking by a pneumatic clamp in a microfluidic channel shows how material can be selectively converted to give different functionalities in the same material piece, even though it is not a single crystal to single crystal conversion, and beckons exploitation for the preparation of systems relevant for molecular electronics as well as other areas where chemical manipulation of single crystals could be beneficial.

Giant reversible nanoscale piezoresistance at room temperature in Sr2IrO4 thin films.

Layered iridates have been the subject of intense scrutiny on account of their unusually strong spin-orbit coupling, which opens up a narrow bandgap in a material that would otherwise be a metal. This insulating state is very sensitive to external perturbations. Here, we show that vertical compression at the nanoscale, delivered using the tip of a standard scanning probe microscope, is capable of inducing a five orders of magnitude change in the room temperature resistivity of Sr2IrO4. The extreme sensitivity of the electronic structure to anisotropic deformations opens up a new angle of interest on this material, with the giant and fully reversible perpendicular piezoresistance rendering iridates as promising materials for room temperature piezotronic devices.

Negative differential resistance (NDR) in similar molecules with distinct redox behaviour.

The transport characterization of self-assembled monolayers (SAMs) based on the closed and open-shell forms of a fully conjugated polychlorotrimethylphenyl (PTM) derivative hybridized with the gold substrate reveals that both systems exhibit negative differential resistance (NDR) in their I-V curves which was attributed to similar resonant tunnelling with unoccupied molecular orbitals. This work demonstrates that distinct transport mechanisms can dominate depending on the bias-voltage applied and shows that NDR processes are not influenced here by the redox character of the molecules.